The present invention generally relates to the field of broadcast antenna designs. More particularly, the present invention relates to the design of a television broadcast antenna that would allow for simultaneous UHF and either low- or mid-band VHF broadcast with equal or less wind load than existing VHF only antennas.
The majority of Ultra High Frequency (UHF) antennas used in National Television System Committee (NTSC) antenna systems are slotted coaxial designs. UHF slotted coaxial antennas gained widespread use in NTSC broadcasting because of their above-average performance characteristics; namely, excellent omni-directional azimuth patterns, low wind loads, and smooth null fill.
While the foregoing performance characteristics are also desirable for digital television (DTV) transmission, the more stringent antenna output performance standards of DTV transmission cannot be met with current slotted coaxial antenna designs. At the present stage of antenna development, the antenna output response performance across multiple channels, which was given little consideration in NTSC systems, is now an important parameter for DTV transmission.
For example, when used as television broadcasting antennas, slotted coaxial antennas are generally optimized to transmit signals for a specified television channel having a six MHz band width. For NTSC transmission, the power distribution across this six MHz band width is concentrated at three basic carrier frequencies; namely, picture, color and aural. Therefore, the performance of the antenna is critical only at these three carrier frequencies.
However, for DTV transmission, the power is equally distributed across a 5.4 MHz frequency span within the six MHz band width. Therefore, the antenna""s performance is critical across substantially the entire operating band. This means that the antenna""s elevation pattern must remain stable (i.e. unchanged) at all frequencies within the band width, and not just at isolated frequencies.
Use of coaxial antennas for DTV transmission is therefore hindered by the fact that slotted coaxial antennas are not suitable for multi-channel applications, such as simultaneous UHF and VHF signaling. This is due in part to the fact that the slots are not broadband radiators.
However, there are over 400 Very High Frequency (VHF) television stations that have already been assigned UHF DTV channels. As a practical matter then, the onset of DTV has thus complicated the antenna selection decision for broadcasters who must now operate VHF/DTV antenna systems simultaneously with their existing UHF/NTSC antenna systems. It would be desirable therefore to provide a replacement antenna that would allow for simultaneous UHF and either low- or mid-band VHF broadcast with equal or less wind load than existing VHF only antennas.
The antenna of the present invention satisfies to a great extent the foregoing need for an improved slotted coaxial antenna design. It combines a DTV channel broadcast system, where the radio frequency signals is in the UHF band, with a NTSC broadcast system, where the radio frequency is in the low and mid VHF band, using a common antenna aperture.
In one aspect of the invention a slotted coaxial antenna constituting a replacement antenna useful for simultaneous UHF and either low- or mid-band VHF broadcast, is provided. The basic design of the replacement antenna involves a marriage between two different types of television broadcast antennas: a slotted coaxial UHF antenna and a circularly polarized VHF antenna.
This slotted coaxial antenna comprises an elongated cylindrical, hollow mast. The mast acts as an outer conductor and support for one or more VHF dipole arms.
On the outside of the mast is arranged a plurality of substantially equidistant, longitudinally extending spaced slots. Each slot is formed in the mast for the purpose of radiating electromagnetic energy. Slots cut in the outer conductor are used for UHF broadcast.
In a preferred embodiment, the VHF antenna of the present invention uses five (5) layers of radiators (i.e. VHF dipole arms) with three radiators mounted symmetrically around the antenna per layer. In other words VHF broadcast, in this instance, is accomplished by employing fifteen substantially equidistantly spaced dipole arms. Alternatively and optionally, seven layers of radiators (i.e. 21 dipole arms) can be used. Each radiator is fed by a single feedline.
On the inside, the mast coaxially surrounds a longitudinally extending UHF inner conductor, which consists of a UHF slotted coaxial antenna. In one embodiment, the UHF inner conductor is surrounded substantially equidistantly by six UHF couplers. Each coupler is located on the inside of the mast between the ends of each longitudinal slot. Four centering pins are used to hold the UHF inner conductor centered within the outer conductor.
A significant result of this slotted coaxial antenna design is an antenna output response performance that is suitable for both low- or mid-band VHF as well as UHF DTV broadcasts.
Another significant result is the achievement of DTV and NTSC signal coverage with equal or less tower wind loading that current VHF only antennas.
As a practical matter, the common aperture UHF/circularly polarized low-and mid-band VHF antenna of the present invention also results in substantial economic savings, since the broadcast of UHF and low/mid-band VHF signals can be transmitted from one common aperture.
There has thus been outlined, rather broadly, the more important features of the invention in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional features of the invention that will be described below and which will form the subject matter of the claims appended hereto.
In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein, as well as the abstract included below, are for the purpose of description and should not be regarded as limiting.
As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention.